The invention relates to an electrooptical device comprising a semiconductor laser. A first radiation path, which terminates at the semiconductor laser, applies an optical signal to the laser. The optical signal influences a parameter of the laser radiation. A second radiation path, which also terminates at the semiconductor laser, provides the laser radiation output.
The parameter to be influenced may be the intensity, the wavelength, or the phase of the semiconductor laser radiation.
Such a device is described, for example, in an article by S. Kobayashi et al entitled "Optical Phase Modulation in an Injection Locked AlGaAs Semiconductor Laser" (IEEE Journal of Quantum Electronics, Vol. QE-18, No. 10, pages 1662-1669, October 1982). The electrooptical device described therein forms part of a so-called coherent optical transmission system which shows great promise for the future because of its large data handling capacity and the large distance between repeater stations.
The described electrooptical device performs optical phase modulation by injecting a coherent radiation beam, originating from, for example, a first diode laser, into a second diode laser which is directly frequency modulated. In practice, this means that in addition to an output transmission fiber, an input transmission fiber, which transmits the radiation from the first diode laser to the second diode laser, must be coupled to the second diode laser of this device.
Apart from being used as a radiation source which is influenced by an injected beam (an injection locked source), a semiconductor laser may be used for various other purposes in coherent optical transmission systems and in other special applications. For example, the laser may be used as a radiation amplifier, an external modulator, a semiconductor laser switch, an active optical starpoint, and a radiation source with external feedback. For these uses, which have been described in the literature, a diode laser must be coupled to an input transmission fiber and at least one output transmission fiber. It has been proposed to couple the transmission fibers to two opposite radiation-emitting surfaces of the semiconductor laser, hereinafter referred to as the front and the rear of the laser. In practice, these fibers must be single mode glass fibers.
As is known, it is very difficult to couple a single mode transmission fiber to the front of a semiconductor laser with the desired coupling efficiency. Coupling a second single mode transmission fiber to the rear of the semiconductor laser will pose even greater practical problems because
(i) this requires the use of a special cooling block for the laser,
(ii) twice as many critical alignment steps must be carried out,
(iii) the package for the semiconductor laser must be redesigned, and
(iv) this package should comprise two fiber seals or two window/lens combinations instead of one.
Moreover, it becomes almost impossible to arrange a detector, for monitoring the laser radiation, inside the package. Further, only two single mode transmission fibers can be coupled to a semiconductor laser.